Fish egg polysialoglycoproteins: circular dichroism and proton nuclear magnetic resonance studies of novel oligosaccharide units containing one sialidase-resistant N-glycolylneuraminic acid residue in each molecule

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Abstract
Long-core units having the common sequence GalNAc.beta.1 .fwdarw. 4(NeuGc2 .fwdarw. 3)GalNAc.beta.1 .fwdarw. 3Gal.beta.1 .fwdarw. 4Gal.beta.1 .fwdarw. 3GalNAc are 1 of the major constituents of rainbow trout egg polysialoglycoproteins. The existing ambiguity regarding the anomeric configuration of the sialidase-resistant unsubstituted sialyl group present in this novel type of oligosaccharide chains has been resolved by a circular dichroism difference spectral method. The fact that the negative band originating from the carbohydrate n .fwdarw. .pi.* transition for this sialyl group was observed offers conclusive proof of the .alpha.-anomeric configuration. The chemical shifts of the sialidase-resistant sialyl H-3eq and H-3ax protons were, respectively, found at relatively higher and lower magnetic field than for the corresponding protons of other sialyl groups. A consideration of molecular models shows that the observed anomalies are all in the directions compatible with expectations on the basis of the magnetic anisotropy effect due to the carboxylate group and steric compression effects by van der Waals interactions between groups that are sterically compressed. In addition to the observed resistance to bacterial sialidases of this sialyl group, it did not behave even as a competitive inhibitor of the sialidase, Arthrobacter ureafaciens, indicating that inaccessibility of this unique sialyl group toward the enzyme. The analysis of the proton nuclear magnetic resonances of sialidase-sensitive mono- and oligosialyl groups present in the long-core units was based on comparisons of diagnostically important regions in the spectra of homologous oligosaccharides of N-glycolylneuraminic acid.